Sakura would be heartbroken. She'd likely spend hours upon hours by Naruto's side, trying to jog his memory with stories of their adventures together. She might even get a bit overbearing in her attempts. Sasuke, who has his own complex relationship with Naruto, would be conflicted. He'd see Naruto's forgetfulness as a sign of weakness at first but then realize the importance of their bond. Kakashi, being the experienced leader, would start by observing Naruto carefully to see exactly how much he has forgotten and then start a systematic process of helping him remember.
At first, Naruto would be extremely hurt. He has always considered Team 7 as his family, so their betrayal would be a huge blow. He might go into a state of shock and disbelief.
He might be really surprised. Naruto is used to his own world and the fanfiction could show things he never imagined. For example, if the fanfiction has his kids achieving great things in new ways, he'd be proud yet a bit shocked at how creative fans can be.
He might be shocked at how the village has developed. In some fanfictions, the village could have advanced technology. Naruto, being from a more traditional ninja world, would likely be both amazed and a bit confused at first.
He might be shocked at first. In fanfiction, the future could be full of unexpected changes. He could be excited to see how his friends and the village have developed. Maybe he'd be a bit sad if things didn't turn out as he hoped for his loved ones.
Sasuke would likely be devastated. Despite their past differences, Naruto was his rival and in a way, his closest friend. He might go into a deep state of regret and isolation. Sakura would be heart - broken. She loved Naruto, and his death would be a huge blow to her. She might become more withdrawn and focus all her energy on her medical ninjutsu. Kakashi would be filled with a sense of failure. He was Naruto's teacher, and he would feel responsible for not being able to protect him.
Sasuke might initially be indifferent on the surface, but deep down he would feel a sense of loss. He has always had a rivalry with Naruto, and Naruto's departure would mean an end to that dynamic. Sakura would be devastated. She has grown to care about Naruto as a friend and teammate, and his leaving would leave a big hole in the team.
They would be shocked at first. Minato would be filled with pride and joy seeing his son all grown up. Kakashi would respect Naruto's strength. Obito might be a bit envious of Naruto's popularity but also admire his spirit. Rin would be very friendly towards Naruto.
The older characters such as Kakashi would be cautious. They would recognize Naruto but also be worried about the implications of someone from the future being in the present. They would want to know if his presence could change the future in a negative way. For example, they might fear that his actions could lead to the earlier awakening of a powerful enemy. As for the villains, they might see future Naruto as a threat and try to eliminate him as soon as possible. Orochimaru, for instance, would be very interested in Naruto's future powers and might try to capture him to study. And the new generation of ninja in the past would look up to future Naruto, seeing him as a symbol of what they could become.